This invention relates generally to fuel cells and, more particularly, to humidification of reactant gases in fuel cells for membrane hydration.
Fuel cells electrochemically convert fuels and oxidants to electricity. A Proton Exchange Membrane (hereinafter xe2x80x9cPEMxe2x80x9d) fuel cell converts the chemical energy of fuels such as hydrogen and oxidants such as air/oxygen directly into electrical energy. The PEM is a solid polymer electrolyte that permits the passage of protons (i.e., H+ions) from the xe2x80x9canodexe2x80x9d side of a fuel cell to the xe2x80x9ccathodexe2x80x9d side of the fuel cell while preventing passage therethrough of reactant fluids (e.g., hydrogen and air/oxygen gases). The direction, from anode to cathode, of flow of protons serves as the basis for labeling an xe2x80x9canodexe2x80x9d side and a xe2x80x9ccathodexe2x80x9d side of every layer in the fuel cell, and in the fuel cell assembly or stack.
In general, an individual PEM-type fuel cell may have multiple, generally transversely extending layers assembled in a longitudinal direction. In a typical fuel cell assembly or stack, all layers which extend to the periphery of the fuel cells have holes therethrough for alignment and formation of fluid manifolds that generally service fluids for the stack. Typically, gaskets seal these holes and cooperate with the longitudinal extents of the layers for completion of the fluid supply manifolds. As may be known in the art, some of the fluid supply manifolds distribute fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) to, and remove unused fuel and oxidant as well as product water from, fluid flow plates which serve as flow field plates of each fuel cell. Other fluid supply manifolds circulate coolant (e.g., water) for cooling the fuel cell.
In a typical PEM-type fuel cell, the membrane electrode assembly (hereinafter xe2x80x9cMEAxe2x80x9d) is sandwiched between xe2x80x9canodexe2x80x9d and xe2x80x9ccathodexe2x80x9d gas diffusion layers (hereinafter xe2x80x9cGDLsxe2x80x9d) that can be formed from a resilient and conductive material such as carbon fabric or paper. The anode and cathode GDLs serve as electrochemical conductors between catalyzed sites of the PEM and the fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) which flow in respective xe2x80x9canodexe2x80x9d and xe2x80x9ccathodexe2x80x9d flow channels of respective flow field plates.
The PEM can work more effectively if it is wet. Therefore, once any area of the PEM dries out, the fuel cell does not generate any product water in that area because the electrochemical reaction there stops. Undesirably, this drying out can progressively march across the PEM until the fuel cell fails completely.
Attempts have been made to hydrate the PEM by raising the humidity of the incoming reactant gases. That is, the fuel and/or oxidant gases are humidified with water, comprising liquid water droplets, for example, atomized droplets from a mist humidifier, before entering the fluid supply manifolds for humidification of the PEM of the fuel cell. In the past it was thought that, once the water entered the active section of the fuel cell, it would evaporate to saturate the reactant gas, promoting hydration of the PEM. However, it is now believed that in such a system, some of the non-evaporated liquid water droplets, having cross sections larger than the fuel channels, actually adhere to the sidewalls of the channels and cause localized flooding of the fuel cell. Also, when water changes phase from a liquid to a gas, energy is required to facilitate this reaction. This is known as the latent heat of vaporization. This absorption of energy causes the temperature of the incoming gases to decrease as some of the water droplets are evaporated, thereby leading to further condensation and flooding, since water will tend to condense from a saturated gas as it cools.
Deleterious effects can also result from turns in the flow path of a stream which is a mixture of liquid water droplets and reactant gas (e.g., two-phase flow). After the stream goes around a given curve, separation of the water from the reactant gas occurs. Anytime the stream changes direction and/or velocity, the various settling rates yield separation. Therefore, by the time the stream reaches the end of such a flow path, much of the liquid water may have settled out. Similar problems and unpredictability can result in any unconstrained flow of water mixed with reactant gas.
Attempts have also been made to introduce a water vapor into the reactant gas streams, for example, by steam injection. Typically this was accomplished by using an external heat source to heat liquid water into vapor. The vapor was then introduced into the reactant gas stream, and into the fuel cell. This process is inefficient since it requires external heat energy to be introduced into the system. Furthermore, the incoming temperature of the reactant gas is high, and this added heat load needs to be removed by the coolant circulating throughout the fuel cell. The increased load on the cooling cycle requires more energy to reduce the temperature of the coolant prior to being re-circulated through the fuel cell. These increased energy requirements are inefficient and costly. Accordingly, it is desirable to provide efficient, adequate hydration to all the fuel cells in the stack.
In an exemplary embodiment of the invention, a system for humidifying a reactant gas for a fuel cell comprises a supply line for supplying reactant gas to a fuel cell, a mist humidifier to produce liquid droplets in the reactant gas, and an evaporator to evaporate the liquid droplets in the reactant gas prior to the reactant gas entering the active section of a fuel cell; the evaporator uses heat removed by the cooling cycle of the fuel cell for evaporation of the liquid droplets.
This system has a number of advantages. The mist humidifier provides a mist of liquid droplets into the reactant gas. The mist is then evaporated by use of discharge heat from the fuel cell. This provides humidified reactant gas to the fuel cell for hydration of the PEM, while reducing the heat reduction load on the fuel cell assembly. As another advantage, the use of excess heat for evaporation of the mist into the reactant gas, serves to decrease the temperature of the coolant, so re-circulation of the coolant back into the fuel cell assembly requires less energy in the coolant re-circulation loop to effect cooling of the fuel cell assembly.